1. Field of the Invention
This invention relates to wireless communication systems.
2. Description of the Related Art
Communication systems, such as wireless systems, are designed to meet various subscriber demands. Service providers continuously seek ways to improve the overall performance of the communication system. In the past, wireless communication systems have been used for voice communications, but more recent technological developments have allowed high speed data transmission as well. As wireless communications become more and more popular for subscribers to obtain data (i.e., e-mail or information from the internet), communication systems should be capable of a higher throughput and be tightly controlled to maintain a high quality of service. Communication is conducted according to any desired communications standard, such as the Universal Mobile Telecommunications Standard (UMTS) or a CDMA standard.
As is known in the art and shown generally in FIG. 1, a wireless communication system 100 serves a service coverage area that is divided into cells 101 having one or more sectors 102. A base station 104 is associated with the sectors 102 in at least one cell 101. Adaptive modulation and coding allows selection of an appropriate transport format (e.g., modulation and coding) for the current channel conditions seen by the user. There are two directions of data flow in such systems; communications from the base station 104 to a mobile device 106 are considered to flow in a downlink direction while the communications originating at the mobile device and sent to the base station are considered to flow in an uplink direction.
As the mobile device 106 moves from one sector 102 to another, the mobile device 106 chooses which base station 104/sector 102 among a set of candidate base stations 104/sectors 102 will provide the best service to it. The mobile device 106 indicates which sector 102/base station 104 it will be served by sending the identity of the chosen sector 102/base station 104 on the uplink. The mobile device 106 may also notify other base stations 104 of its chosen base station 104/sector 102. The resource metrics needed to communicate with the mobile device 106 (e.g. data packets) are then transferred to the chosen base station 104/sector 102, and then data is sent from the chosen base station 104/sector 102 down to the mobile device 106.
The actual protocol used by the mobile device 106 to indicate its desire to switch to a particular base station 104/sector 102 among a group of candidate base stations 104/sector 102 and denote its intent to switch to the chosen base station 104/sector 102 at a particular time is specified according to the applicable communications standard (e.g., CDMA Revisions C and D) used by the system 100. The switching protocol in these current communications standards can function effectively only if the network infrastructure (which comprises base stations and additional controlling entities, such as radio network controllers) reliably detects the mobile device's indication of a switch. One such mechanism would be to monitor the mobile's transmissions continuously for a switch indication at the base station 104 with which it is currently in communication. Once the mobile device's switch indication is detected at this base station 104/sector 102, it may transfer data resources to the new base station/sector selected by the mobile device 106. However, because the link between the mobile device 106 and the current base station 104/sector 102 is weakening (i.e., the newly selected base station 104/sector 102 has a better link to the mobile device 106 than the current base station 104/sector 102), it is possible that the current base station 104/sector 102 may miss the switch indication from the mobile device 106.
Although more complex algorithms may be executed in a central entity, such as a radio network controller 116, to increase sector switch detection reliability, such algorithms require a large amount of data to be transferred from the base station to the central entity, which does the actual switch detection calculation and determination. This introduces a great deal of processing delay. Moreover, the limited bandwidth between the base stations and the central entity as well as the limited processing power at the central entity itself makes more complex algorithms an unrealistic solution for improving switch detection reliability.
There is a desire for a mechanism that will reliably detect the sector switch indication from the mobile device 106 and accurately estimate the time at which the mobile device 106 will switch from one sector 102 to another. There is also a desire to conduct reliable sector switch detection without requiring large amounts of data to be transferred from the base stations to the central entity to evaluate whether the mobile device 106 has actually sent a sector switch indication.